A tissue is said to be differentiated when it has begun to synthesize characteristic macromolecules specific for that tissue. In some tissues, specific proteins have been well characterized and their appearance during histodifferentiation can be monitored and assayed in minute quantities. In keratinizing epithelim, the pattern of appearance of constitutive proteins during differentiation has been monitored in experiements with skin, but this approach to the study of oral epithelium has not been made. In these experiments, we have analyzed the dorsal tongue epithelium of the rat during development by morphological and biochemical methods. A serial study of the epithelium as it differentiates from the 12th fetal day to mature keratinizing filiform papillae was done by electron microscopy. In the same age groups, the constitutive proteins of the lingual epithelium were analyzed biochemically with SDS-polyacrylamide gel electrophoresis which yielded quantitative and qualitative information on the proteins important in keratinization and characterized them according to apparent molecular weight. For each developmental stage, the protein was correlated with morphological findings. In current experiments, the same types of morphological and biochemical analyses are being applied to epithelia harvested from organ cultures of fetal tongue mucosa grown for up to 8 days and from epithelia of developing cheek and skin. The major significance of this work is that it could form the basis for a biochemical classification of both heathy and diseased tissues by their constitutive protein profiles. BIBLIOGRAPHIC REFERENCE: Baratz, R.S. and Farbman, A.I. 1975. Morphogenesis of rat lingual filiform papillae. Am. J. Anat. 143:283-302.